The sLHC would be a souped-up LHC. If all goes to plan, it will come online in around a decade after upgrades. The beams would be 10 times as bright, which would involve increasing the number of protons in each beam by a factor of 10, and result in 10 times as many collisions per hour. This means the sLHC has a greater chance of seeing an interesting collision but also has more uninteresting collisions to filter through, as well as more radiation for the detector to withstand. To cope with these challenges, the beam injectors will be replaced, additional superconducting magnets will control the brighter beams, and detectors will be upgraded to cope with the higher data rates and radiation doses.

Date of completion: 2018
Cost: $1.27 billion
Pros: Much cheaper than building a new machine
Cons: Challenging environment to make precision measurements; only small increase in particle masses probed

The International Linear Collider

If the project receives financial backing after technical reports due in 2012, the ILC would be a 35-kilometre-long straight accelerator. While the LHC collides protons, which contain quarks and gluons, the ILC will smash electrons and positrons. Collisions will be “cleaner” than the LHC because electrons and positrons are fundamental particles. This makes for less ambiguity when trying to work out what produced any new particles. In the LHC, charged particles lose energy with each rotation.

Date of completion: 2020s
Cost: $8 billion
Pros: Cleaner collisions. Technology reliable and well understood
Cons: In some scenarios, the maximum energy may not be sufficient to see all new physics of interest

The Compact Linear Collider

The CLIC would be a positron and electron linear accelerator like the ILC - and is also yet to be approved - but it would be shorter and have collisions at higher energies. A high-intensity, low-energy drive beam runs parallel to the colliding beams. Power built up in the drive beam is transferred in quick bursts to the main beams.

Date of completion: 2020s
Cost: No official estimate, ~$10 billion
Pros: Cleaner collisions. High energy and compact - the ILC would need to be 140 kilometres long to achieve the same energy, so vastly more expensive. Greater sensitivity to massive particles compared with sLHC
Cons: R&D for the new technology is still at an early stage

Far in the future

Other proposals include the Very Large Hadron Collider, which would have a collision energy of 40 to 200 TeV and would have to be built from scratch. Muon colliders, and an LHeC - smashing an electron beam into a proton beam - are also being considered.